1. Field of the Invention
The present invention concerns an MR tomography with a system for contrast optimization of MRT.
2. Description of the Prior Art
Magnetic resonance tomography (MRT) enables the non-invasive acquisition of measurement data of a patient and processing the acquired measurement data into high-contrast images that are used in medical diagnostics. Various pulse sequences are available for MR imaging that differ due to the different type of the echo generation (spin echo, gradient echo) and the adjustment of the relevant measurement parameters (such as the repetition time TR and the echo time TE of the flip angle α etc.) within a selected pulse sequence.
Tissue-specific parameters are the spin-grid relaxation time T1 and the spin-spin relaxation time T2 as well as the proton density PD. Based on the tissue-dependent specification of these parameters, it is possible to image tissue with different contrast in an MR image via suitable selection of the repetition time TR and the echo time TE. Depending on the selection of TR and TE, T1-weighted, T2-weighted or proton-weighted MR images can be acquired. Which of these weightings is the most suitable (i.e. delivers the most high-contrast MR image) depends on the diagnostic goal, in particular on the type of the examined tissue.
The contrast can additionally be influenced by the application of contrast agent or by preparation of the magnetization, such as through MTC (magnetization transfer contrast).
Examples for pulse sequences that differ with regard to the contrast of the obtainable MR images, or within which the MR image contrast can be varied significantly by variation of the measurement parameters TR, TE and α, are the HASTE sequence, the TrueFISP sequence, the DESS sequence, the CISS sequence and the SINOP sequence. Depending on the selection of the above parameters, T1-or T2-weighted MR images are obtained in which watery structures or fatty structures are shown brightly contrasting.
The HASTE sequence delivers a weakly T2-weighted MR image for a relatively small effective echo time Teff, i.e. for a short temporal interval between the excitation pulse and the echo in which the phase coding gradient has the smallest amplitude, while T2-weighted MR images are obtained for a long effective echo time Teff. The multi-echo HASTE sequence enables the acquisition of MR images with varying contrast. With regard to the contrast of different MR images, these can be further used for the generation of a combination image that exhibits a better contrast in an anatomical region of interest.
Gradient echo sequences lead to mixed contrasts. The TrueFISP sequence thus delivers a typical T2/T1 contrast. The cerebrospinal fluid is shown very bright while tissue (such as white or grey brain matter) appears only with weak signal in the MR image. The DESS sequence comprises the simultaneous acquisition of an MR image on the basis of the aforementioned FISP sequence and on the basis of a further MR image using the PSIF sequence, which (presented in a simplified manner) represents a reversal of the TrueFISP sequence. While the FISP sequence shows the typical T2/T1 contrast, the PSIF MR image is strongly T2-weighted. An MR image with particularly high brightness in the region of cerebrospinal fluid is acquired via the subsequent addition of the magnitudes of the images in the framework of the DESS method, i.e. the addition of the signal intensities of pixels at the same location. The 3D DESS method is therefore primarily used for orthopedic imaging, for example for the differentiation of cartilage and fluid.
The CISS sequence includes two successive 3D b-SSFP passes and is thus likewise based on the TrueFISP sequence. Two image data sets are acquired whose “banding” artifacts are displaced counter to one another. The “banding-free” CISS image is obtained via “maximum intensity projection” or a special algorithm for combination of these two image data sets.
It is known to add or to subtract two congruent images generated in the same manner, i.e. images that image the same section in the examination subject. For each image point the magnitude (absolute) value of the associated image point of the second image is added to or subtracted from a magnitude value of an image point of a first image. An existing contrast can be intensified (amplified) in this manner, or new contrasts can be generated. Moreover, the image addition or the image subtraction offers the possibility to avoid image artifacts. The CISS sequence is implemented with the goal of the avoiding of such artifacts.
The post-processing of measurement data (such as raw data or image data) occurs using known algorithms such as, for example, the sum-of-squares algorithm.
Image subtraction for this purpose is known, for example, from DE 196 16 387 A1. A further method for image generation is described in DE 101 21 802 A1 (corresponding to United States Application Publication No. 2002/0183612 A1). These latter, corresponding publications disclose a magnetic resonance tomography apparatus having a system for contrast optimization of MRT images, having an input unit for measuring parameters such as, for example, TR, TE, FOV, matrix size, flip angle, etc., a unit for establishing the anatomical area in the examination subject to be examined, a unit for combining a number of MRT images acquired with various measurement parameters, and a unit for visualization and generation of a DICOM header for selected MRT image combinations.
DE 196 16 387 A1 concerns what is known as the HIRE method (“High Intensity Reduction Sequence”). After an excitation, two groups of nuclear magnetic resonance signals are acquired in two time spans with varying interval for excitation. The image is acquired via subtraction of the nuclear magnetic resonance signals of the first and the second group with respectively coinciding spatial coding.
DE 101 21 802 A1 concerns a method for image generation that comprises the generation of a first image matrix and of a second image matrix and the addition or subtraction of the magnitude values of the image points with the same spatial coding in both image matrices. In this method the magnitude values of the image points are weighted dependent on the local image conditions, with the image points usable for an improvement of the image quality being filtered out. This method for image post-processing is applied to image matrices that were acquired according to the aforementioned HIRE method or the DESS method.
In WO 2004/095048 A1 a method for generation of a magnetic resonance image is disclosed in which a number of echo signals are acquired with at least two different echo time values. The echo signals of each time value are then processed into an intermediate image, after which an analysis of the intermediate image and the ultimate combination into a total image follow.
The possibilities described above for contrast improvement, contrast generation and avoidance of artifacts in MR images represent a considerable enrichment of MR technology. At the same time, however, they also place ever greater demands on the user because an increasing, unmanageable number of parameters must be taken into account to solve presented tasks, such as the high-contrast or contrast-intensified imaging of a body region and the achievement of the contrast optimal for the diagnostic use. The correct adjustment of these parameters can increasingly only be effected by highly qualified technicians. Moreover, measurements in a magnetic resonance tomograph are relatively costly, such that in the application case it is not possible to achieve the desired result iteratively under implementation of a plurality of preliminary tests on a patient. The implementation of such preliminary tests is also precluded if the patient (as often occurs) is not able or willing to remain motionless for longer times in a magnetic resonance tomography scanner.
The introduction of imaging systems into the clinical routine is assisted by flexible software that is user-friendly. The SYNGO® platform (registered trademark of the Siemens Corporation) represents such software with a user-friendly user interface (Reichert T., Herget M., “SYNGO—The new standard for viewing and workstation software”; electromedica, 1999, 67(2): p. 60-63).